Fixes #31024 - Optimize host taxonomy_conditions (#8101)

Previously, `Taxonomy.expend` was used to get the taxonomies for the
user or the current taxonomy, then all taxonomies were iterated over
to get the subtree ids.
However, when `Taxonomy.current` is `nil`, we already get
all subtree ids since `expend` calls `my_taxonomy` which return the
subtrees already. Instead, we can directly call the
`taxonomy_and_subtree_id` method to get the ids. In the case of admin
user, we don't need a taxonomy condition for `nil` taxonomies as that
should return all taxonomies.
When `Taxonomy.current` is present, we can call the `subtree_ids` method
on it directly without needing to wrap it in an array and iterate over
it.
In the case of `nil` current taxonomies, this can potentially save
hundreds of useless database queries to check subtree ids for all the
taxonomies a user has access to. For example, in my setup every call to
`/api/hosts/$id` triggered 20 queries per taxonomy the user is assigned
to (due to the permission section of the `show.json.rabl` view).